(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of an air-fuel ratio in an internal combustion engine having two air-fuel ratio sensors upstream and downstream of a catalyst converter disposed within an exhaust gas passage.
(2) Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio.
According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three-way reducing and oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NO.sub.X simultaneously from the exhaust gas.
In the above-mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed at a location near the concentration portion of an exhaust manifold, i.e., upstream of the catalyst converter, the accuracy of the controlled air-fuel ratio is affected by individual differences in the characteristics of the parts of the engine, such as the O.sub.2 sensor, the fuel injection valves, the exhaust gas recirculation (EGR) valve, the valve lifters, individual changes due to the aging of these parts, environmental changes, and the like. That is, if the characteristics of the O.sub.2 sensor fluctuate, or if the uniformity of the exhaust gas fluctuates, the accuracy of the air-fuel ratio feedback correction amount FAF is also fluctuated, thereby causing fluctuations in the controlled air-fuel ratio.
To compensate for the fluctuation of the controlled air-fuel ratio, double O.sub.2 sensor systems have been suggested (see: U.S. Pat. Nos. 3,939,654, 4,027,477, 4,130,095, 4,235,204). In a double O.sub.2 sensor system, another OP.sub.2 sensor is provided downstream of the catalyst converter, and thus an air-fuel ratio control operation is carried out by the downstream-side O.sub.2 sensor in addition to an air-fuel ratio control operation carried out by the upstream-side O.sub.2 sensor. In the double O.sub.2 sensor system, although the downstream-side O.sub.2 sensor has lower response speed characteristics when compared with the upstream-side O.sub.2 sensor, the downstream-side O.sub.2 sensor has an advantage in that the output fluctuation characteristics are small when compared with those of the upstream-side O.sub.2 sensor, for the following reasons:
(1) On the downstream side of the catalyst converter, the temperature of the exhaust gas is low, so that the downstream-side O.sub.2 sensor is not affected by a high temperature exhaust gas.
(2) On the downstream side of the catalyst converter, although various kinds of pollutants are trapped in the catalyst converter, these pollutants have little affect on the downstream side O.sub.2 sensor.
(3) On the downstream side of the catalyst converter, the exhaust gas is mixed so that the concentration of oxygen in the exhaust gas is approximately in an equilibrium state.
Therefore, according to the double O.sub.2 sensor system, the fluctuation of the output of the upstream-side O.sub.2 sensor is compensated for by a feedback control using the output of the downstream-side O.sub.2 sensor. Actually, as illustrated in FIG. 1, in the worst case, the deterioration of the output characteristics of the O.sub.2 sensor in a single O.sub.2 sensor system directly effects a deterioration in the emission characteristics. On the other hand, in a double O.sub.2 sensor system, even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, the emission characteristics are not deteriorated. That is, in a double O.sub.2 sensor system, even if only the output characteristics of the downstream-side O.sub.2 are stable, good emission characteristics are still obtained.
In the above-mentioned double O.sub.2 sensor system, for example, an air-fuel ratio feedback control parameter such as a rich skip amount RSR and/or a lean skip amount RSL is calculated in accordance with the output of the downstream-side O.sub.2 sensor, and an air-fuel ratio correction amount FAF is calculated in accordance with the output of the upstream-side O.sub.2 sensor and the air-fuel ratio feedback control parameter (see: U.S. Ser. Nos. 831,566 and 848,580). As explained above, since the downstream-side O.sub.2 sensor has low response speed characteristics, the mean air-fuel ratio on the upstream-side O.sub.2 sensor is already deviated greatly from the stoichiometric air-fuel ratio to the rich side when the output of the downstream-side O.sub.2 sensor is switched from the lean side to the rich side. Thus, the HC and CO emissions are increased. Similarly, the mean air-fuel ratio on the upstream-side O.sub.2 sensor is already deviated greatly from the stoichiometric air-fuel ratio to the lean side when the output of the downstream-side O.sub.2 sensor is switched from the rich side to the lean side. Thus, the NO.sub.x emission is increased. In the double O.sub.2 sensor system, however, the renewal speed .DELTA.RS of the air-fuel ratio feedback control parameter is always definite, regardless of the output of the downstream-side O.sub.2 sensor, and accordingly, the catalytic cleaning rate .eta. as shown in FIG. 2 is not considered. That is, as shown in FIG. 2, when the controlled air-fuel ratio is deviated from a window W defined by an optimum cleaning rate .eta..sub.0 to the lean side, the cleaning rate .eta. of the NO.sub.X emission is remarkably reduced. Note that, when the controlled air-fuel ratio is deviated from the window W to the rich side, the cleaning rates of the HC and CO emissions are also reduced, however, this reduction is smaller than the above-mentioned reduction of the cleaning rate of the NO.sub.X emission.
Also, in order to improve the emission characteristics, it is possible for the renewal speed of the air-fuel ratio feedback control parameter to be increased regardless of the output of the downstream-side O.sub.2 sensor, however, this does not make effective use of a double O.sub.2 sensor system and creates a problem in that the controlled air-fuel ratio is rapidly changed, thus reducing the drivability. Further, in order to reduce the NO.sub.X emission, it is also possible for the exhaust gas recirculation (EGR) to be increased; the compression ratio reduced; or the ignition timing retarded, however, this invites combustion malfunctions, thus also reducing the drivability.